added doc for isatty()
diff --git a/Doc/lib/libos.tex b/Doc/lib/libos.tex
index cb55eb5..aa8c7d5 100644
--- a/Doc/lib/libos.tex
+++ b/Doc/lib/libos.tex
@@ -360,6 +360,12 @@
Availability: \UNIX{}.
\end{funcdesc}
+\begin{funcdesc}{isatty}{fd}
+Return \code{1} if the file descriptor \var{fd} is open and connected to a
+tty(-like) device, else \code{0}.
+Availability: \UNIX{}
+\end{funcdesc}
+
\begin{funcdesc}{lseek}{fd, pos, how}
Set the current position of file descriptor \var{fd} to position
\var{pos}, modified by \var{how}: \code{0} to set the position